Kotlin multiplatform + Domain models = ❤ Nico Krijnen Niels Marsman 

Nico Krijnen & Niels Marsman Nico Krijnen Niels Marsman 

Moving fast & Focus on innovation It's all about 

What is a domain model? Need to look at business logic Decisions made by code that are specific to the domain of the app Cargo shipping Airline booking Education Insurance Medical research 

What is a domain model? Need to look at business logic Decisions made by code that are specific to the domain of the app Cargo shipping Airline booking Education Insurance Medical research 

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UI HTML / CSS Application Layer Data Access Layer Navigation Offline Storage 

Routes Services Data Access 

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Routes Services Data Access UI HTML / CSS Application Layer Data Access Layer Navigation Offline Storage Where is the business logic? 

Routes Services Data Access UI HTML / CSS Application Layer Data Access Layer Navigation Offline Storage Where is the business logic? 

class LegoSetRentalService(private val legoSetRentalDAO: LegoSetRentalDAO) { // . .. fun rent(set: LegoSet, duration: Duration) { var price = set.basePrice price *= when (set.condition) { SetCondition.MINT -> 1.0 SetCondition.GOOD -> 0.9 SetCondition.FAIR -> 0.75 SetCondition.POOR -> 0.5 } price *= when { duration.days >= 30 - > 0.8 duration.days >= 7 -> 0.9 else -> 1.0 } legoSetRentalDAO.insert(LegoSetRental(set, duration, price)) } } Where is the business logic? 

class LegoSetRentalService(private val legoSetRentalDAO: LegoSetRentalDAO) { // . .. fun rent(set: LegoSet, duration: Duration) { var price = set.basePrice price *= when (set.condition) { SetCondition.MINT -> 1.0 SetCondition.GOOD -> 0.9 SetCondition.FAIR -> 0.75 SetCondition.POOR -> 0.5 } price *= when { duration.days >= 30 - > 0.8 duration.days >= 7 -> 0.9 else -> 1.0 } legoSetRentalDAO.insert(LegoSetRental(set, duration, price)) } } Where is the business logic? 

class LegoSetReturnService( ... ) { fun returnSet(set: LegoSet, damageReport: DamageReport) { requireNotNull(set.currentRental) { "Set is not currently rented" } val condition = when { damageReport.missingPieces > set.totalPieces.value * 0.05 -> SetCondition.POOR damageReport.missingPieces > 0 - > SetCondition.FAIR else - > set.condition } val status = when { condition = = SetCondition.POOR - > RentalStatus.MAINTENANCE else - > RentalStatus.AVAILABLE } legoSetRentalDAO.insert(LegoSetReturn(set, condition, status)) } } Where is the business logic? 

class LegoSetReturnService( ... ) { fun returnSet(set: LegoSet, damageReport: DamageReport) { requireNotNull(set.currentRental) { "Set is not currently rented" } val condition = when { damageReport.missingPieces > set.totalPieces.value * 0.05 -> SetCondition.POOR damageReport.missingPieces > 0 - > SetCondition.FAIR else - > set.condition } val status = when { condition = = SetCondition.POOR - > RentalStatus.MAINTENANCE else - > RentalStatus.AVAILABLE } legoSetRentalDAO.insert(LegoSetReturn(set, condition, status)) } } Where is the business logic? 

class LegoSetReturnService( ... ) { fun returnSet(set: LegoSet, damageReport: DamageReport) { val condition = SetCondition.by(set, damageReport) val status = RentalStatus.by(condition) legoSetRentalDAO.insert(LegoSetReturnRequest(set, condition, status)) } } sealed interface SetCondition { data object Mint : SetCondition data object Good : SetCondition data object Fair : SetCondition data object Poor : SetCondition companion object { fun by(set: LegoSet, damageReport: DamageReport): SetCondition = when { damageReport.missingPieces > set.totalPieces.value * 0.05 -> Poor damageReport.missingPieces > 0 -> Fair else -> set.condition } } } Where is the business logic? 

class LegoSetReturnService( ... ) { fun returnSet(set: LegoSet, damageReport: DamageReport) { val condition = SetCondition.by(set, damageReport) val status = RentalStatus.by(condition) legoSetRentalDAO.insert(LegoSetReturnRequest(set, condition, status)) } } sealed interface SetCondition { data object Mint : SetCondition data object Good : SetCondition data object Fair : SetCondition data object Poor : SetCondition companion object { fun by(set: LegoSet, damageReport: DamageReport): SetCondition = when { damageReport.missingPieces > set.totalPieces.value * 0.05 -> Poor damageReport.missingPieces > 0 -> Fair else -> set.condition } } } class LegoSetReturnService( ... ) { fun returnSet(set: LegoSet, damageRe requireNotNull(set.currentRental val condition = when { damageReport.missingPieces > -> damageReport.missingPieces > - > else - > set.condition } val status = when { condition = = SetCondition.PO - > else - > RentalStatus.AVAILAB } legoSetRentalDAO.insert(LegoSetR } } Where is the business logic? 

Domain model? Code that makes your app unique and deals with your domain Routes Services Data Access UI HTML / CSS Application Layer Data Access Layer Navigation Offline Storage 

Domain model? Domain model = only 20% of your code Routes Services Data Access UI HTML / CSS Application Layer Data Access Layer Navigation Offline Storage 

Model = Simplification Each useful to solve a specific problem 

Domain Model Stub Stub Stub Domain Model Domain Model Domain Model 

Domain Model Domain Model Domain Model Domain Model Stub Stub Stub 

Kotlin Multiplatform! 

Kotlin Multiplatform! 

Kotlin Multiplatform! Kotlin Compiler JVM JavaScript Android TypeScript (preview) Android NDK iOS watchOS tvOS macOS Windows Linux WebAssembly 

Domain Model Domain Model Domain Model Domain Model Domain Model 

Ingredients of succes 

Domain Model = Encapsulated 

Hexagonal / Union Architecture 

Dependencies only point inward 

✅ Pure Kotlin + Stdlib ❌ JVM apis or libraries ❌ JS apis or libraries ❌ Native apis or libraries Domain model = Free of dependencies 

Hexagonal / Union architecture ✅ Isolated, pure-code domain model ✅ No framework code mixed in ✅ Easy to read and understand ✅ Easy to test ✅ Always-valid Immutable 

In action 

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@Composable private fun LegoSetDetailScreen( legoSet: LegoSet, onBackClick: () -> Unit, onRentClick: (Duration) -> Unit, modifier: Modifier = Modifier ) { Scaffold( topBar = { TopAppBar( title = { Text("Set Details") }, navigationIcon = { IconButton(onClick = onBackClick) { Icon(Icons.AutoMirrored.Filled.ArrowBack, "Back") } }, actions = { IconButton(onClick = { legoSet.toggleFavorite() }) { Icon( if (legoSet.isFavorite) Icons.Filled.Favorite else Icons.Filled.FavoriteBorder, "Favorite" ) } } ) } ) .. . Kotlin / UI 

Column( modifier = Modifier .padding(16.dp) .fillMaxWidth() ) { Box( modifier = Modifier .fillMaxWidth() .height(300.dp) ) { HorizontalPager( state = pagerState, contentPadding = PaddingValues(bottom = 0.dp), modifier = Modifier.fillMaxSize() ) { page -> // Image Carousel } } } Text( text = legoSet.name, style = MaterialTheme.typography.h3, fontWeight = FontWeight.Bold ) Spacer(modifier = Modifier.height(8.dp)) Row( verticalAlignment = Alignment.CenterVertically ) { Text( text = "Set #${legoSet.id} • Ages ${legoSet.ageRecommendation}", style = MaterialTheme.typography.body1 ) } Kotlin / UI 

class LegoSet( .. . ) { fun calculateRentalPrice(duration: Duration): Money { . .. } fun returnSet(missingPieces: Int, damageReport: String? = null) { . .. } fun canBeRentedTo(age: Int, duration: Duration): RentalEligibility { . .. } fun performMaintenance(replacedPieces: Int) { . .. } } Kotlin / Domain Model 

fun calculateRentalPrice(duration: Duration): Money { var price = basePrice / 7.0 * duration.days.toDouble() // Condition affects price val conditionMultiplier = when (condition) { SetCondition.MINT - > 1.0 SetCondition.GOOD - > 0.9 SetCondition.FAIR - > 0.75 SetCondition.POOR - > 0.5 } price = price * conditionMultiplier // Duration discounts price = when { duration.days >= 30 - > price * 0.8 // 20% discount for monthly rentals duration.days >= 7 -> price * 0.9 // 10% discount for weekly rentals else -> price } // Premium for high-value sets if (replacementValue > USD(200.0)) { price = price * 1.2 } return price } Kotlin / Domain Model 

fun canBeRentedTo(age: Int, duration: Duration): RentalEligibility { if (status != RentalStatus.AVAILABLE) { return RentalEligibility.NotEligible( "Set is currently ${status.name.lowercase()}") } if (AgeRecommendation(age) < recommendedAge) { return RentalEligibility.NotEligible( "Set requires minimum age of ${recommendedAge}") } if (condition == SetCondition.POOR && duration.days > 7) { return RentalEligibility.NotEligible( "Sets in poor condition can only be rented for up to 7 days") } return RentalEligibility.Eligible() } Kotlin / Domain Model 

data class Duration private constructor(val days: Int) { init { require(days >= 0) { "Duration cannot be negative" } } companion object { fun days(days: Int) = Duration(days) fun weeks(weeks: Int) = Duration(weeks * 7) } val weeks: Int get() = days / 7 } enum class SetCondition { MINT, GOOD, FAIR, POOR } // Result type for rental eligibility check sealed class RentalEligibility { data class Eligible(val message: String = "Rental is eligible") : RentalEligibility() data class NotEligible(val reason: String) : RentalEligibility() } Kotlin / Domain Model 

Let's do some live coding! 

Can we re-use more? 

as reached Beta! o becoming Stable now! Single codebase for the application NativeCode SharedCode Businesslogic andcore View iOS specificAPIs Android specificAPIs View Learn more about beta → Case studies Support Get started Kotlin Multiplatform Mobile is an SDK for iOS and Android app development. It offers all the combined benefits of creating cross-platform and native apps. It is trusted in production by many of the world’s leading companies, including Philips, Netflix, Leroy Merlin, and VMWare. Singl logic Maintain a other logic Implement devices. Busines andcor iOS spec We loved the “shared business, native UI” idea that Kotlin Multiplatform promoted, and it m our teams did not have to give up using their preferred toolchains.” “ Get started Case studies Kotlin Multiplatform 

Kotlin Multiplatform Mobile is an SDK for iOS and Android app development. It offers all the combined benefits of creating cross-platform and native apps. It is trusted in production by many of the world’s leading companies, including Philips, Netflix, Leroy Merlin, and VMWare. Singl logic Maintain a other logic Implement devices. Busines andcor iOS spec We loved the “shared business, native UI” idea that Kotlin Multiplatform promoted, and it m our teams did not have to give up using their preferred toolchains.” “ Get started Case studies kmp.jetbrains.com 

Organize for success! 

Organize for success! 

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🚨 🚨 🚨 🚨 🚨 🚨 

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Kotlin Isolated ❤ ❤ ❤ Kotlin multiplatform 🤔 Not for everyone 👬👫 Team organization matters Domain models Key Takeaways 

Kotlin multiplatform + Domain models = ❤ 

Kotlin multiplatform + Domain models = ❤ Nico Krijnen Niels Marsman